This invention is concerned with wastewater treatment plants, particularly with secondary clarifiers and with removal of settled solids via rake arms operating along the bottom of the clarifier.
In a sewage treatment plant, the rate of removal of settled solids in secondary clarifiers can be critical. Most plants have nutrient removal permits, especially regarding phosphorus. The rate of removal of the solids in such plants is important. Settled solids should be removed quickly to prevent them from surfacing after going anaerobic and releasing phosphorus into the wastewater, which can impact the phosphorus permit limit for the plant.
Most conventional clarifier sludge rake arms have blade flights and squeegees, designed to travel along the floor, with a design configuration that can require two or three rake arm revolutions to cause migration of engaged sludge outwardly to a point of removal at a sludge pickup tube. Such pickup tubes draw the sludge upwardly to a elevated sludge return box in a hydraulic differential sludge removal clarifier. If settled sludge in the clarifier basin dwells in the basin too long, anaerobic conditions are created in the sludge blanket, resulting in release of phosphorus into the wastewater rather than removal of the phosphorus from the basin, as noted above. The phosphorus can then float to the surface of the clarifier and potentially exit the clarifier over the influent weirs. As explained above, this can impact the phosphorus permit limit of the treatment plant. Extremely important in plants with strict nutrient limits, this applies as well to and is useful in all secondary clarifiers in municipal wastewater treatment plants.
A typical prior art conventional clarifier sludge removal system included rake arms with blade flights and attached squeegees of straight configuration, mounted at oblique angles to the length of the rake arm. A series of three or four of these are arranged parallel to one another, spaced apart so as to act as a comb, simply pushing or urging the settled sludge outwardly in the clarifier by an incremental distance with each revolution. In this way, sludge moved by a flight in one pass is engaged by a farther-out flight in the next pass of a rake arm (which will be the opposing arm). If four blade flights were positioned between the clarifier center and a sludge pickup tube, then in theory at least four rake arm passes (at least two full revolutions) would be required to move the innermost sludge to the position of the pickup tube. At the pickup tube would be a pair of such flights in a V-formation with the open end forward, tending to collect sludge that has been fed out to that distance from center. Several series of such obliquely angled flights have typically been included on a rake arm, one for each pickup tube location.
As noted above, the requirement of two or three revolutions of the slowly moving rake arms can be objectionable when the plant is limited as to phosphorus discharged in the effluent water from the plant. The objective of the invention is to reduce the number of rotations of the rake arm to bring sludge to pickup points and thus to reduce the dwell time of settled sludge within the clarifier.